Question: A man can row three-quarters of a kilometre against the stream in 11 minutes and down the stream in 7 minutes. The speed (in km/hr) of the man in still water is:
[A].2
3
4
5
[A].
[B].
[C].
[D].
Answer: Option D
Explanation:
We can write three-quarters of a kilometre as 750 metres,
and 11 minutes as 675 seconds.
| Rate upstream = | 750 | m/sec | = | 10 | m/sec. | |
| 675 | 9 |
| Rate downstream = | 750 | m/sec | = | 5 | m/sec. | |
| 450 | 3 |
| Rate in still water = | 1 | 10 | + | 5 | m/sec | |
| 2 | 9 | 3 |
| = | 25 | m/sec |
| 18 |
| = | 25 | x | 18 | km/hr | |
| 18 | 5 |
= 5 km/hr.